The present invention relates to an ignition apparatus for an internal combustion engine having a current limiting function in which a switch such as a power transistor for turning on and off the power supply to an ignition coil to thereby generate a high voltage can be protected from damage due to an overcurrent flowing therethrough. More particularly, it relates to such an ignition apparatus which is simple in construction and low in the manufacturing costs.
FIG. 3 shows a typical example of an ignition apparatus for a multi-cylinder (e.g., four-cylinder in the illustration) internal combustion engine having a current limiting function. In this figure, the illustrated ignition apparatus includes a power supply in the form of a battery 1, and a plurality of ignition coils 2 provided one for each of cylinders of the engine and commonly connected to the battery 1. Each ignition coil 2 has a primary winding and a secondary winding. The primary winding of each ignition coil 2 is connected at one end thereof to the battery i and at the other end thereof to ground through a switch 3 in the form of an electronic switch comprising a pair of transistors coupled in a Darlington pair. The secondary winding of each ignition coil 2 connected to a spark plug (not shown) for a corresponding cylinder. Each switch or Darlington pair 3 has a common collector coupled to the primary winding of a corresponding ignition coil 2, an emitter connected to ground, and a base connected to an unillustrated engine control unit which periodically supplies thereto an ignition signal D at an appropriate ignition timing whereby the switch 3 is turned off, thus generating a high voltage Vi across the secondary winding which is imposed upon electrodes of a corresponding spark plug.
A current limiting circuit, generally designated at reference numeral 9, is provided for each of the switches 3 for the purpose of protecting it from an overcurrent flowing therethrough. Each current limiting circuit 9 Includes a current sensing resistor 5 having a limited resistance and connected between the emitter of a corresponding switch 3 and ground for sensing a current flowing from the battery 1 to ground through the primary winding of a corresponding ignition coil 2 and the switch 3 to generate a corresponding voltage Vt thereacross which is proportional to the current flowing through the switch 3. An operational amplifier 7 has a positive or non-inverted input terminal connected to one end (i.e., a switch-side end) of the current sensing resistor 5 to which the corresponding switch S is connected, and a negative or inverted input terminal connected to a positive terminal of a reference .voltage supply 6 whose negative terminal is connected to the other end (i.e., a ground-side end) of the resistor 5 which is connected to ground. The reference voltage supply 6 generates a reference voltage Vr corresponding to an upper limit value of a current flowing through the switch S and supplies it to the negative input terminal of the operational amplifier 7, so that the operational amplifier 7 compares the voltage Vt across the resistor 5 with the reference voltage Vr to generate an output signal or voltage A proportional to a difference (Vt-Vr) therebetween. A transistor 8 has a base coupled to an output terminal of the operational amplifier 7, a collector coupled to the base of a corresponding switch 3, and an emitter connected to ground. Thus, when the output voltage A from the operational amplifier ? exceeds a predetermined threshold value, the transistor 8 is turned on by the output signal A from the operational amplifier 7 to connect or shore-circuit the base of the switch 3 to ground through the now conductive transistor 8, thereby turning the switch 3 off to stop the current flow through the primary winding of the corresponding ignition coil 2 and the switch 3.
The operation of the above-mentioned ignition apparatus of FIG. 3 will now be described. When an ignition signal D supplied from the unillustrated engine control unit to a switch S is changed into a high level at an appropriate power supply starting timing to thereby turn the switch 3 on, a current begins to flow from the battery 1 to ground through the primary winding of an associated ignition coil 2, the switch 3 and an associated resistor 5. As a result, a voltage Vt develops across the resistor 5 and is supplied to the positive or non-inverted input terminal of the operational amplifier 7. The operational amplifier 7 compares the voltage Vt with the reference voltage Vr which is supplied from the reference power supply 6 to the negative or inverted input terminal of the amplifier 7, and it then generates an output signal A proportional to a difference (Vt-Vr) between the voltages Vt, Vr so as to control an associated transistor 8, For example, when the voltage Vt exceeds the reference voltage Vr (i.e., Vt &gt;Vr) indicating that an overcurrent flows through the switch 3, the operational amplifier ? operates to increase the level or magnitude of its output signal A. When the output signal A from the operational amplifier 7 exceeds a predetermined threshold, the transistor 8 is made conductive so that the ignition signal D from the engine control unit is allowed to pass to ground via the now conductive transistor 8 while bypassing the switch 3. As a result, the base current to be supplied to the base of the switch 3 is suppressed. With such control of the base current, the current flowing through the switch 3 is limited to a predetermined upper limit level.
Accordingly, the current passing through each switch 3 during conduction or energization of a corresponding ignition coil 2 does not exceed the upper limit value, and hence each switch 3 is positively protected from damage due to an overcurrent passing therethrough.
In this connection, it is to be noted that in each current limiting circuit 9, a circuit constant of the resistor 5 for generating a high current sensing voltage Vt thereacross and a circuit constant of the reference voltage supply 6 are properly adjusted in advance.
Subsequently, when the ignition signal D supplied to the switch 3 becomes low at ignition timing to turn the switch 3 off, a high voltage Vi is induced across the secondary winding of a corresponding ignition coil 2 and supplied to an illustrated corresponding spark plug to thereby ignite a corresponding cylinder.
With the above-described ignition apparatus as constructed above, however, the current limiting circuits 9 are provided one for each ignition coil 2 and hence for each cylinder, so the number of the current limiting circuits required is equal to that of the cylinders, making the overall construction of the ignition apparatus complicated. As a result, it is difficult to manufacture the apparatus at reduced costs. Moreover, the resistance of the resistor 5 and the reference voltage vr for comparison with the voltage across the resistor 5 in each current limiting circuit 9 have to be respectively adjusted, which results in variations in the values of currents limited by the respective current limiting circuits 9.
Moreover, in order to detect whether a cylinder to be controlled has actually ignited, an unillustrated ignition detecting circuit may be provided as necessary. For example, such an ignition detecting circuit is constructed such that it generates an ignition detection signal to the unillustrated engine control unit when a voltage corresponding to a primary winding current flowing through the primary winding of an ignition coil 2 is less than a predetermined lower limit value required for proper ignition. Based on this ignition detection signal, the engine control unit determines whether the ignition coil 2 operates in a normal manner. If the voltage-corresponding to the primary winding current is less than the predetermined lower limit value, the engine control unit determines that the ignition coil 2 is abnormal, and it takes an appropriate measure such as stopping fuel injection to a cylinder corresponding to the abnormal ignition coil 2.
In this case, however, the ignition detecting circuit is provided independently and separately from the current limiting circuit 9, so the provision of the independent and separate ignition detecting circuit result in a further complicated construction and a further increase in the manufacturing cost.